A martial arts black belt who became a morphine-dependent invalid after a beating by a senior-ranking police officer, has won a five-year battle for compensation.

Christchurch man Russell Owen Byfield, a black belt in karate and a pro ballroom dancer, was left seriously injured after an attack by Senior Sergeant Ron Greatorex on May 6, 2005.

Greatorex was in an unmarked car, and not in uniform, when he flashed his lights to get Byfield to stop his SUV.

When Byfield stopped at a railway barrier, Greatorex, wearing blue police-issue overalls without any official police insignia, approached him and told him to get out of his car. Byfield demanded to see formal police identification. Greatorex was unable to provide him with any.

Byfield, assuming Greatorex was just a workman, and fearing a possible road-rage incident, drove off. Greatorex followed him home and delivered what witnesses described as a violent beating.

A witness said Greatorex punched Byfield "approximately 15 times . . . in the face, neck and body area but mainly his neck".

"He shredded Russell's clothing from neck to groin. This man was clearly out of control."

Byfield suffered numerous injuries in the assault, including friction burns to his upper and lower arm, upper and lower-arm contusion, chest contusion, and contusion of the scalp and neck. He still suffers from neck pain and numbness in his arms.

Greatorex then charged Byfield with failing to stop for a police officer, assaulting an officer and escaping custody.

Byfield was initially convicted, but that was quashed two years later on appeal to the High Court at Christchurch.

Justice John Fogarty said he was "quite satisfied" that the overalls Greatorex was wearing were "not sufficiently distinctive" to identify him as a policeman and Byfield was entitled to drive off.

He said Byfield was also within his rights to tell Greatorex - who he still did not realise was a police officer - to stay off his property and even entitled to use "reasonable force to prevent him trespassing on his property".

The judge said: "When trying to make an arrest, the police officer was not acting in the execution of his duty. He had no right to be in the drive at the gate in the first place."

He threw the case out and told police not to attempt a retrial.

"I am satisfied that on the facts of this case there is no possibility of the police obtaining convictions on these charges."

Byfield said he was left with such severe injuries after the assault he has still not recovered.

Last week Accident Compensation Corporation (ACC) was ordered to pay Byfield an undisclosed sum to cover weekly compensation backdated to 2008, according to a decision released by the Auckland District Court.

Greatorex has never been charged and still works as a senior-ranking police officer in Christchurch.

At the time of the assault, Greatorex was a senior member of the police staff safety tactical training team in Christchurch.

Police said Greatorex was investigated in 2011, but that "none of the complaints made by Mr Byfield were upheld", despite a High Court ruling quashing the man's convictions.

Byfield told the Sunday Star-Times the assault had been "horrific" and he was still taking pain medication for a permanent neck injury and he was "in a lot of pain and not sleeping well at all".

ACC had refused to compensate Byfield because a medical expert, who had not examined him, claimed the injuries to his neck were degenerative and did not stem from the incident, but this claim was dismissed by Judge Roderick Joyce who relied on evidence from doctors who had examined him.

After the incident, Byfield was given assistance with paying for physiotherapy. He applied to ACC for weekly compensation in 2008 after the pain meant he was not able to continue working as a mechanic or run his diving business.

The application was declined, but that decision has now been over-ruled.